1. Field of the Invention
The present invention relates to a servo valve control system for a hydraulically actuated control member.
Hydraulically actuated control members and control systems incorporating such members, which may be a hydraulic jack or the like, are well known and have found usage in virtually all types of apparatus. Basically, these systems involve a control valve, which controls the flow of the hydraulic fluid to/or from the hydraulic jack and a servo valve which controls the position of the control valve by regulating a servo pressure applied to the positioning elements of the valve.
Servo valve controls are known which include a movable blade disposed at the end of a movable rod which extends through an opening in a spring tube which is fixed, at one end, on a base and which is free at its opposite end. The rod is supported by a transverse element which is movable through one or more electrical drive coils. When a current energizes the drive coils, the blade moves between a pair of nozzles which are each connected to a control chamber of the control valve so as to vary the pressure within the control chambers and, consequently, change the position of the control valve.
Although such devices have been relatively effective, they have the serious drawback of being very sensitive to any pollution present in the servo valve fluid. The small diameter of the servo valve nozzles coupled with the small clearances between the nozzles and the blade (which may be several hundredths of a mm) allows any particles to jam the blade and prevent operation of the servo valve. These devices also suffer from the relative fragility of the spring tube caused by the vibration filled environment in which such devices are typically utilized. The breakage of such spring tube would, of course, render the servo valve totally inoperative.
One way of remedying these defects in the known servo valves, has been to include a plurality of them in the control system to provide a redundancy in case one or more should fail. Since most servo valves have double electrical windings it is relatively easy to accomplish the electrical redundancy that is necessary. However, it is much more difficult to provide two or more servo valves to control the same member insofar as it requires a system of hydraulic isolation to isolate the defective servo valve from the member being controlled. Without isolating the malfunctioning servo valve, it prevents the other servo valves from fully performing their operative function.
British Pat. No. 1,369,441 discloses an electrohydraulic control system wherein two parallel servo valves receive the same electrical control signals and simultaneously act on the same pressure chambers of an actuating member. Means are provided to detect an operational difference between the two servo valves and to generate an error signal. However, in case one of the servo valves malfunctions, the only actions made possible by such a system are either locking the actuating member in position or activating an emergency control to move the member into a specific position, or isolating one of the servo valves by means of a hydraulic switch slide.
U.S. Pat. No. 3,437,312 to Jenny describes a redundant control system which includes three computation channels associated with three comparators and two servo valves. The two servo valves are separated by a hydraulic selector which admits to the controlled hydraulic member fluid from the first servo valve and subsequently fluid from the second servo valve.